1. Field of the Invention
The present invention relates to a nitrogen oxide removal control apparatus for reducing nitrogen oxides involved in exhaust gas by controlling injection of a reducing material such as ammonia into the exhaust gas from a gas turbine of a power generation plant.
2. Description of the Prior Art
Power generation plants using a gas turbine include a simple cycle power generation plant where a generator is driven by at least one gas turbine for power generation, and a combined cycle power generation plant where a plurality of gas turbines and at least one steam turbine are provided, the steam turbine being driven by steam produced by making use of heat of exhaust gas from the gas turbines, and electric power is generated by these turbines.
The combined cycle power generation plant of the so-called waste heat recovery system is divided roughly into a single shaft type and a multiple shaft type. In the single shaft type, a gas turbine, a steam turbine, and a generator are coupled with a common shaft, and in the multiple shaft type a gas turbine and a steam turbine have separate individual shafts to each of which generators are coupled.
In such power generation plants, a fuel from a fuel system is supplied to a combustor together with air from a compressor and is combusted, and produced combustion gas is supplied to a gas turbine. The gas turbine is driven with the combustion gas, whereby a generator coupled on the same shaft is rotated to provide an electric power.
Waste gas exhausted from a gas turbine generally involves nitrogen oxides called NOx as a general term such as NO and NO.sub.2, etc., harmful to human bodies. The NOx promotes pollution of environment as it is discharged to the air, so that it should be removed before it is exhausted to the atmosphere.
For this, the best efforts are paid by providing a nitrogen oxide removal apparatus, which is capable of reducing NOx concentration by introducing a reducing material into a flue for guiding exhaust gas.
The nitrogen oxide removal apparatus is to reduce and decompose nitrogen oxides to harmless nitrogen gas and water vapor by introducing a reducing material such as ammonia into exhaust gas and thereafter forcing the exhaust gas to pass through a catalyst on the downstream side. In the nitrogen oxide removal process, non-reacted NOx is exhausted intactly into the atmosphere. When ammonia is used as the reducing material, for example, the non-reacted NOx is increased with too little supply of the ammonia, while the non-reacted ammonia is exhausted with too much supply of the same. The process suffers from responsibility of the control, which is not necessarily satisfactory, because of its being a chemical reaction of NOx and ammonia in the presence of a catalyst. In the nitrogen oxide removal apparatus, there are produced varieties of inconveniences if any reducing material of the amount matching to that of NOx involved in exhaust gas is not supplied at proper timing, so that there is required a nitrogen oxide removal control apparatus as one for controlling the supply amount and supply timing of the reducing material.
It is noted that standards of NOx exhausted from the plant are applied for flow rates or concentrations thereof, and for both of them. The nitrogen oxide removal control apparatus is therefore needed to control a control target such that the NOx flow rate, the NOx concentration or any severe one therebetween satisfys the standards. When the NOx concentration is regulated, for example, the nitrogen oxide removal control apparatus executes feedback control such that a measured NOx from a detector for measuring the NOx concentration at an outlet of the nitrogen oxide removal apparatus becomes equal to a NOx setpoint set to be less than a standard value.
Such feedback control has no problem provided the amount of NOx produced in a combustor is less varied. Only with the feedback control, however, the control target fails to be satisfactorily controlled upon the plant being started or stopped or upon a plant load being varied owing to a fact that the amount of produced NOx is violently varied with changing load, owing to a fact that the nitrogen oxide removal process is delayed, and owing to a fact that a measurement by a NOx detector at the outlet of the nitrogen oxide removal apparatus is delayed. To solve this difficulty, prior art discloses techniques for compensating the change in the NOx production with the change in the load by incorporating feedforward control additionally to the feedback control.
For example, U.S. Pat. No. 4,473,536 and U.S. Pat. No. 4,473,537 disclose a control system wherein a mole ratio of ammonia/NOx is estimated by proportional integration (PI) control based upon a deviation of a setpoint and a measured value of NOx, which ratio is in turn multiplied by a predicted amount of NOx at an inlet of the nitrogen oxide removal apparatus estimated from the operating conditions of a gas turbine to yield an ammonia flow rate to be injected. In the control system, the mole ratio of ammonia/NOx is multiplied by the predicted amount of NOx, so that a loop gain is lowered as the predicted amount of NOx is reduced. Since the predicted amount of NOx may involve error caused by computation accuracy, a control signal perhaps involves error correspondingly.
Increasing concern is recently concentrated to natural environment destruction, and varieties of regulations to NOx are provided and become progressively severe. Additionally, the nonreacted reducing material such as, for example, ammonia has been rendered to varieties of regulations.
For this, there have been developed the so-called low NOx combustors which produce less NOx compared with conventional combustors. Such low NOx combustors are classified according to combustion techniques typically into a multistage combustor and a catalyst combustor, between which combustors with the multistage combustion technique are practically promising.
Such multistage combustion technique is to restrict production of NOx by supplying a fuel successively dividing the fuel longitudinally of a combustor to lower flame temperature. In the multistage combustion technique, however, produced NOx concentration upon starting has a peak every time the fuel is changed over, the peak tending to be higher than that in a conventional combustor. The NOx production of the multistage combustor is regulated to be lower than that of the conventional combustor in the vicinity of a rated load, but in order to reduce the amount of NOx from the multistage combustor lower than the standards there is also required a nitrogen oxide removal apparatus.
In the case where a produced NOx amount is varied rapidly with a greater extent as in a power generation plant possessing a gas turbine unit of the multistage combustion technique, the prior art fails to control a NOx amount at an outlet of the nitrogen oxide removal apparatus to a predetermined value following up to change in a plant state with high accuracy. The bad controllability requires injection of an excessive reducing material, resulting in poor economy and a factor of environment destruction.